The present invention relates to a light emitting diode, and more particular to the light emitting diode and a method of fabrication thereof in which an intermediate layer is disposed between a confining layer and a window layer to improve the boundary characteristic therebetween, so that the defect of the high density caused by lattice mismatch between the confining layer and the window layer can be decreased.
AlGaInP is the chemical compound of III-V group materails and the direct-transition material having the wide energy band. This materials is used for the light emitting diode having the frequency band of approximately 560-680 nm. In AlGaInP, the frequency is dependent upon the ratios of the Ga and Al, and generally the frequency become short as the ratio of Al is increased.
The AlGaInP light emitting diode used in the various display devices is shown in FIG. 1. As shown in FIG. 1, the typical AlGaInP light emitting device is fabricated by growing the a plurality of the epitaxial layers on a n-GaAs semiconductor substrate. First, the n-AlGaInP is epixatially grown on the semiconductor substrate 1 to form a first confining layer 3a. On the first confining layer 3a, the (AlxGa1xe2x88x92x)0.5In0.5P is epitaxially grown to form an activation layer 5 which generates the light having the desired frequency. A second confining layer 3b of p-type is disposed on the activation layer 5 by growing the p-AlGaInP epitaxially. On the rear side of the LED, i.e., the rear surface of the semiconductor substrate 1, a first electrode 7a including a conductive metal, which is called a rear side electrode, is formed. Further, a second electrode 7b, front side electrode, is formed in a part of the front surface of the LED, the light emitting area.
In this LED, the current applied from the second electrode 7b should be dispersed in the edge direction of the LED chip and flowing uniformly through the p-n conjunction area of the n-type confining layer 3a and the p-type confining layer 3b. AlGaInP layer used as the uppermost layer of the LED such as the p-type confining layer has the high resistance because of the low hole mobility caused by the limitation of the p-dopant level. In AlGaInP layer, thus, the current tends to flow locally into the lower portion of the second electrode 7b, not dispersed in the edge direction sufficiently. This called xe2x80x98current crowding phenomenonxe2x80x99. By the current crowding phenomenon, the most of light that is emitting in the outside of the LED is generated under the opaque second electrode 7b. As a result, the light to be emitted in the outside of the LED is blocked by the opaque electrode 7b, so that the emitting efficiency is deteriorated.
In order to overcome the light-blocking problem, some method have been introduced. One method of these is to form the front electrode of the grid shape in the entire front area of the LED. Since the current is applied from the electrode covering the entire area of the LED, it is uniformly flowing through the entire p-n junction. In this LED, however, the opaque electrode is also blocking the light, so that the light emitting efficiency may be decreased.
Other method is to use the transparent electrode as the front electrode of the LED. The transparent electrode includes oxidation layer such as Indium Tin Oxide(ITO). However, since the ITO has the high resistance, the resistance of the LED may be increased.
Another method is disclosed in U.S. Pat. No. 5,008,718. In this patent, the window layer including the low resistance characteristic such as AlGaAs, GaAsP, and GaP is deposited on the confining layer. Since the bandgaps of these layers are larger than that of the AlGaInP light emitting leyer, the light emitted from the emitting layer is transparent therethrough. This method is described in FIG. 2.
The LED shown in FIG. 2 has the identical structure with the LED shown in FIG. 1 except the window layer 59. Since the materials such as AlGaAs, GaAsP, and GaP have the high electron mobility(i.e., low resistance), the current applied from the electrodes 7a,7b is easily flowing into the edge region. Therefor, the current crowding phenomenon is prevented and the efficiency of the device can be improved.
In this method, however, lattice mismatch phenomenon is generated at the boundary, because the lattice constant of the GaP window layer 9 is different from that of the AlGaInP confining layer. By the lattice mismatch, the strain is generated between the confining layer 3 and the window layer 9 and the defect is increased in the window layer 9. Accordingly, the reliability and the optical characteristic of the device is deteriorated. Further, when the window layer 9 is made of the materials including AlGaAs and AlGaInP, the ratio of Al should be increased to transmit the light through the window layer 9 although the lattice is matched between the layers. Thus, the resistance of the window layer 9 may be increased and light having wavelength lower than 600 nm cannot be transparent effectively.
In order to solve the defect increasing problem, double window layers is introduced in U.S. Pat. No. 5,359,209. In this method, it is very difficult to remove perfectly the lattice mismatch phenomenon, so that the forward voltage Vf is insufficiently decreased because of the defect at the boundary. In addition, when the material such as GaAs is used as the window layer, it""s bandgap is smaller than that of the AlGaInP, so that the window layer is operated as a light absorption layer and as a result the brightness of the device may be decreased.
It is an object of the present invention to provide a light emitting diode and a method of fabricating thereof in which the light emitting diode includes an intermediate layer made of non-single crystalline material between a confining layer and window having single crystalline layer to decrease the defect caused by the lattice mismatch and improve the brightness and forward voltage characteristics.
In order to achieve the object, the light emitting diode according to the present invention includes a semiconductor substrate, a first confining layer made of n-AlGaInP on the semiconductor substrate, an activation layer made of AlGaInP on the first confining layer, a second confining layer made of p-AlGaInP on the activation layer, a window layer including an intermediate layer made of non-single crystalline material on activation layer, the window layer being made of single crystalline material having the larger bandgap and lowr resistance than the activation layer, the intermediate layer being disposed at the boundary of the activation layer to improve the boundary characteristic so that the defect generated at the boundary is decreased, and first and second electrodes disposed respectively on the semiconductor substrate and the window layer.
The window layer includes material selected from the group consisting of p-Gap, GaAsP, and GaxIn1xe2x88x92x, where 0.7xe2x89xa6xxe2x89xa61. The intermediate layer in the window layer is formed in the thickness of approximately 0.01-0.5 xcexcm and the window layer is formed in approximately 5-15 xcexcm.
Further, the method of fabricating the light emitting diode according to the present invention comprises the steps of providing a semiconductor layer, forming activation p-n junction layer made of AlGaInP on the semiconductor layer, the p-n junction layer for emitting the light, forming the intermediate layer on the p-n junction layer to decrease the lattice mismatch between the single crystalline material, the intermediate layer being made of non-single crystalline material, forming the window layer on the intermediate layer, the window layer including the material having the larger bandgap and the lower resistance than the p-n junction layer, and forming respectively the first and second electrodes on the semiconductor layer and the window layer.
All the layers may be deposited with the MOCVD process. The intermediate layer and the window layer are deposited with same process. First, the material selected from the group consisting of p-GaP, GaAsP, and GaxIn1xe2x88x92xP is deposited at the first temperature of 400-700xc2x0 C. Thereafter, these materials are deposited at the same condition thereon during rising the temperature to the second temperature higher than the first temperature.